1. Field of the Invention
The present invention relates to a structure for positioning a cam shaft on a cylinder head of an engine.
2. Description of the Related Art
In some engine employed in an automobile, a motorcycle, a small ATV (small all terrain vehicle), a snow mobile, a small leisure vehicle, a personal watercraft, or the like, a single cam shaft is mounted on a cylinder head. The cam shaft serves to operate an intake/exhaust valve of the engine by means of a rocker arm and is positioned so that a cam face thereof is at a proper position with respect to the rocker arm.
FIG. 8 is a view showing the conventional cam shaft positioning structure and a longitudinal sectional view of a cylinder head 140 on which a rocker case 150 is mounted. A cam shaft 120 is provided with a flange portion 131. The flange portion 131 is axially positioned by a groove 149 formed in the cylinder head 140. The width (axial dimension) of the groove 149 is slightly larger than the thickness (axial dimension) of the flange portion 131. Therefore, the flange portion 131 is substantially undisplaceable in the axial direction because of restriction of the groove 149. Thus, the cam shaft 120 is axially positioned with respect to the cylinder head 140 so as to be placed at a normal position.
The rocker case 150 is fixed on the cylinder head 140. The structure for axially positioning the cam shaft 120 is not provided in the rocker case 150. For example, a groove 159 is formed in the rocker case 150 and the width (axial dimension) thereof is considerably larger than the thickness of the flange portion 131. Therefore, the flange portion 131 is not in contact with an inner wall face of the groove 159.
However, in the cam shaft positioning structure of FIG. 8, a procedure for placing the cam shaft 120 on the cylinder head 140 and then mounting the rocker case 150 on the cylinder head 140 takes long time and impedes an assembly process of the engine.
In this procedure, before the rocker case 150 is mounted on the cylinder head 140, the cam shaft 120 is placed on the cylinder head 140. At this time, the flange portion 131 of the cam shaft 120 is fitted into the groove 149 of the cylinder head 140. At this stage, a cam sprocket 115 is not mounted to the cam shaft 120 yet.
Subsequently, with a chain 163 put around the cam sprocket 115, the cam sprocket 115 is mounted to the cam shaft 120. The cam sprocket 115 is secured to the cam shaft 120 by means of two bolts.
Then, the rocker case 150 provided with a rocker arm is mounted on the cylinder head 140. Thereby, the mounting of the cam shaft 120 is completed.
In the above-described procedure, the operation in which the chain 163 is put around the cam sprocket 115 which is then secured to the cam shaft 120 by means of the bolts makes the inefficient assembly and therefore impedes the assembly process.
If the cam sprocket 115 is mounted to the cam shaft 120 in advance and then the chain 163 is put around the cam sprocket 115, it becomes impossible to fit the cam shaft 120 into the cylinder head 140. The reason for this is that since the thickness of the flange portion 131 is substantially equal to the width of the groove 149, and therefore, there is little play between the flange portion 131 and the groove 149, the flange portion 131 cannot be inclined with respect to the groove 149 when inserted thereinto.
The present invention addresses the above-described conditions, and an object of the present invention is to provide a cam shaft positioning structure of a single overhead cam type engine capable of providing efficient assembly.
To achieve the above-described object, according to the present invention, there is provided a cam shaft positioning structure of an engine comprising: a cylinder head provided with one part of a bearing; a rocker case provided with the other part of the bearing; and a cam shaft rotatably supported by the bearing formed by mounting the rocker case on the cylinder head, and being applied to a single over head cam type engine, wherein the cam shaft is provided with a flange portion, the rocker case is provided with a groove for accommodating the flange portion, and wherein in a first state in which the rocker case is mounted on the cylinder head and the cam shaft is securely retained between the cylinder head and the rocker case, the groove is adapted to restrict axial displacement of the flange portion to allow the cam shaft to be axially positioned with respect to the cylinder head so as to be placed at a normal position, and in a second state in which the cam shaft is placed on the cylinder head and the rocker case is not mounted on the cylinder head, the cam shaft is able to be axially displaceable with respect to the cylinder head from the normal position.
In this constitution, with the cam shaft placed on the cylinder head, the cam shaft is axially displaceable. Therefore, after mounting the cam sprocket and the like to the cam shaft, the cam shaft can be placed on the cylinder head in an inclined condition and a chain can be put around the cam sprocket in a loose condition. In addition, the axial positioning of the cam shaft can be performed by mounting the rocker case to the cylinder head later.
In the cam shaft positioning structure of an engine, it is preferable that a guide portion for guiding the flange portion into the groove is formed by cutting out the rocker case at both end portions in a circumferential direction of the groove. With this constitution, the alignment of the groove and the flange portion can be easily made by the guide portion.
It is preferable that the cam shaft positing structure of an engine, comprises: restricting means for restricting axial displacement of the cam shaft with respect to the cylinder head from the normal position so as to be within a predetermined range in the second state, wherein, in the second state, when the cam shaft is axially displaced most greatly to one side of the cylinder head, a position of one end face of the flange which is close to one side of the cylinder head is closer to the other side of the cylinder head than a first axial position, the first axial position corresponds to one end position of an entrance of the guide portion which is close to the one side of the cylinder head in the first state, in the second state, when the cam shaft is axially displaced most greatly to the other side of the cylinder head, a position of the other end face of the flange is closer to one side of the cylinder head than a second axial position, and the second axial position corresponds to the other end position of an entrance of the guide portion in the first state. With this constitution, the flange portion is guided into the groove by the guide portion regardless of whether the cam shaft is axially displaced most greatly to one side or to the other side of the cylinder head.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.